1. Field
The present invention relates to a thin film solar cell, and more particularly, to a see-through-type thin film solar cell and a method of manufacturing the same.
2. Description of the Background Art
Solar cell refers to a semiconductor device for directly converting solar energy into electricity. The solar cell can be mainly classified into a silicon-based solar cell, a compound-based solar cell, and an organic solar cell depending on its use material.
The silicon-based solar cell is in detail classified into a single crystalline silicon solar cell, a polycrystalline silicon solar cell, and an amorphous silicon solar cell depending on a semiconductor phase.
The solar cell is classified into a bulk (substrate) solar cell and a thin film solar cell depending on a semiconductor thickness. In the thin film solar cell, a semiconductor layer has a thickness of to several μm to several tens of μm.
In the silicon-based solar cell, the single crystalline and polycrystalline silicon solar cells belong to the bulk solar cell. The amorphous silicon solar cell belongs to the thin film solar cell.
The compound-based solar cell is classified into the bulk solar cell based on gallium arsenide (GaAs) and indium phosphide (InP) of groups III-V, and the thin film solar cell based on cadmium telluride (CdTe) of groups II-VI and copper indium diselenide (CIS) (CuInSe2) of groups I-III-VI. The organic solar cell is mainly classified into an organic molecular solar cell and an organic/inorganic complex solar cell. In addition, there is a dye-sensitized solar cell. They all belong to the thin film solar cell.
Among several kinds of solar cells, the bulk silicon solar cell having a high-energy conversion efficiency and a cheap manufacturing cost is being popularly utilized for ground power.
In recent years, there is a trend in which a raw material increases in price because of its shortage as a demand for the bulk silicon solar cell suddenly increases. A thin film solar cell whose silicon raw material can reduce by one several hundredth of the present time is being greatly required for the development of a technology for low pricing and mass production of a solar cell for massive ground power.
FIG. 1 is a diagram illustrating a structure of a unit cell of a conventional integrated thin-film solar cell. FIG. 2 is a diagram illustrating an example of a conventional integrated thin-film solar cell.
As shown in FIGS. 1 and 2, a conventional integrated thin-film solar cell 1 is of unit cell structure in which a plurality of unit cells 20 connect in series on a glass substrate or transparent plastic substrate 10 (hereinafter, referred to as “transparent substrate”).
In the integrated thin-film solar cell, the unit cell includes a transparent electrode 22 mutually cut (insulated), and formed in a band shape on the transparent substrate 10 that is an insulator; a unit solar cell (semiconductor) layer 24 covering the transparent electrode 22 and formed in a band shape; and a metal back electrode layer 26 covering the solar cell layer 24 and formed in a band shape. The integrated thin-film solar cell is of structure in which the plurality of unit cells 20 connect in series with each other. The metal back electrode is covered with a back protective layer 30 formed of resin for preventing electrical short-circuiting of the solar cell and protecting the solar cell.
In general, a laser patterning method, a chemical vaporization machining (CVM) method, a metal probe-based mechanical scribing method are being used to manufacture the integrated thin-film solar cell 1.
The laser patterning method refers to a technology for etching the transparent electrode 22, the solar cell (semiconductor) layer 24, and the metal back electrode layer 26, mainly using YAG laser beam. A detailed usage method will be described as follows.
FIG. 2 illustrates the example of the conventional integrated thin-film solar cell.
As shown in FIG. 2, the transparent electrode 22 is formed on the transparent substrate 10, and is etched using laser beam under the atmosphere. After that, the solar cell (semiconductor) layer 24 is formed and is cut (insulated) using the laser beam under the atmosphere. The metal back electrode layer 26 is formed and is etched using the laser patterning process under the atmosphere, thereby connecting the solar cells in series and forming the unit cell of the integrated solar cell.
A drawback of the laser patterning method will be described below.
As shown in FIG. 2, the transparent electrode 22 is formed on an entire upper surface of the transparent substrate 10. After that, the transparent electrode 22 is cut in the laser patterning method, and is cut (insulated) in the band shape having a predetermined width. Then, the cut transparent electrode 22 has a width of 50 μm to several 100 μm in general.
After that, a process of forming the solar cell (semiconductor) layer 24 is performed mostly under vacuum whereas the laser patterning for cutting the solar cell (semiconductor) layer 24 is performed under the atmosphere. This makes it impossible to perform a sequential process under the vacuum, thereby deteriorating an operation efficiency of a manufacturing device. This inevitably results in a price increase for the solar cell. Also, there is a drawback that the unit cell is deteriorated in characteristic due to adherence of moisture and a contaminant because the substrate is exposed to the atmosphere to etch the solar cell layer 10.
Next, the metal back electrode layer 26 is formed under the vacuum by a sputtering method and is again laser-patterned under the atmosphere, thereby manufacturing the unit cell of the integrated solar cell. This process can cause process discontinuity and contaminant drawbacks as described above. An ineffective area (cut width) between the unit cells 20 of the solar cell increases through a total of three-times laser patterning including two times of laser patterning for cutting the transparent electrode 22 and the solar cell (semiconductor) layer 24, and one time of laser patterning for cutting the metal back electrode layer 26 and concurrently connecting the solar cells in series. Thus, a loss of an effective area of the solar cell increases. There is a drawback that a laser patterning equipment is expensive, and a precision position control system is required for patterning at an accurate position, thereby increasing a manufacturing cost.
The chemical vaporization machining method refers to a technology for simultaneously cutting the solar cell (semiconductor) layer into the plurality of unit cells having a uniform width, by locally generating atmospheric pressure plasma around line electrodes that have diameters of tens of μm and are arranged in a grid form in proximity to an upper portion of a substrate, using SF6/He gas.
The chemical vaporization machining method has a feature of short process time, excellent film selectivity, and less film damage compared with the laser patterning method. The chemical vaporization machining method has an advantage of preventing a performance of the solar cell from being deteriorated by the exposure of the substrate to the atmosphere because etching is performed under the vacuum unlike the laser patterning method, and reducing the manufacturing cost compared with the laser patterning method.
However, the precision position control system capable of accurately controlling a position within a vacuum device is needed because the etching should be performed in an accurate position adaptively to the patterned transparent electrode. This is of very difficult matter when the solar cell is manufactured using a large-scale substrate. A gap obtained by the etching is about 200 μm to the minimum, and is greater than an insulation gap obtained using the laser patterning method. Thus, there is a drawback of increasing the loss of the effective area of the solar cell.
Another etching method is the mechanical scribing method. This method makes it possible to perform collective scribing, by a plurality of metal probes, correspondingly to the number of necessary unit cells, and is greater in extensibility and adaptability to high-speed processing than the laser patterning method. The mechanical scribing method refers to an etching method in which device and operation costs are most cheap compared with the above two methods.
In a CIS solar cell for example, a CdS/CIS layer is being popularly used to manufacture the CIS solar cell because it is softer than molybdenum (Mo), thereby facilitating scribing based on the scribing method.
However, the conventional mechanical scribing method has a drawback that it needs the laser patterning equipment for etching a back electrode (Mo) and a front electrode (ZnO), and the precision position control device for accurately controlling the position because it is limitedly used only for the solar cell (semiconductor) layer.